Evaluating Grafted Watermelon for Verticillium Wilt Severity, Yield, and Fruit Quality in Washington State

Abstract

<jats:p>Verticillium wilt caused by <jats:italic>Verticillium dahliae</jats:italic> is a serious disease for watermelon growers in Washington State. Grafting represents a possible alternative disease management strategy, but little is known about rootstock resistance to verticillium wilt or the performance of grafted watermelon in the different production regions of the state. In this study, verticillium wilt severity, yield, and fruit quality were evaluated at three contrasting field sites in Washington using verticillium wilt-susceptible ‘Sugar Baby’ (diploid) watermelon grafted onto four commercial rootstock cultivars (Marvel, Rampart, Tetsukabuto, and Titan); nongrafted ‘Sugar Baby’ was included as the control. <jats:italic>Verticillium dahliae</jats:italic> soil densities varied at each site (&lt;1.0, 5.7, and 18.0 colony-forming units (cfu)/g soil at Othello, Eltopia, and Mount Vernon, respectively). Area under disease progress curve (AUDPC) values differed significantly among treatments at Eltopia and Mount Vernon. Nongrafted ‘Sugar Baby’ had the highest AUDPC value at all three sites, while ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ had the lowest AUDPC value at Eltopia and Mount Vernon. Nongrafted ‘Sugar Baby’ also had the lowest fruit weight per plant at all sites, but ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ had the highest fruit weight per plant at Eltopia and Mount Vernon. Marketable fruit weight per plant did not differ among treatments at Othello. Yield was negatively correlated with AUDPC values at both Eltopia and Mount Vernon. Fruit number per plant was only significantly impacted at Eltopia, where ‘Sugar Baby’ grafted onto ‘Tetsukabuto’ had more fruit per plant than all other treatments except ‘Sugar Baby’ grafted onto ‘Rampart’. Fruit quality (flesh firmness, total soluble solids, and lycopene content) was unaffected by grafting at either Othello or Eltopia, except for increased flesh firmness for ‘Sugar Baby’ grafted onto ‘Marvel’ and ‘Titan’ as compared with nongrafted ‘Sugar Baby’ at Eltopia. At season’s end, plants were sampled from all treatments at Eltopia and Mount Vernon and assayed for <jats:italic>V. dahliae</jats:italic>. Microsclerotia typical of this organism were observed in all samples. Results from this study indicate that verticillium wilt of watermelon can be successfully managed by grafting when the <jats:italic>V. dahliae</jats:italic> soil density exceeds 5.0 cfu/g in Washington. In addition, grafting does not reduce fruit quality and using certain rootstocks can improve the quality of flesh firmness at certain locations.</jats:p>

Cucurbitaceae Germplasm Resistance to Verticillium Wilt and Grafting Compatibility with Watermelon

“…Experimental controls consisted of two commercial cultivars, rootstock cv. Tetsukabuto (C. maxima · C. moschata) (American Takii, Inc., Salinas, CA) as the verticillium wilt-resistant control (Wimer et al, 2015a) and diploid (seeded) watermelon cv. Sugar Baby (Osborne Quality Seeds, Mount Vernon, WA) as the verticillium wilt-susceptible control.…”

“…Grafting compatibility can be defined as the successful union of the rootstock and scion in a manner that does not compromise fruit yield and quality. Previous studies have found that yield of watermelon grafted onto interspecific Cucurbita hybrids and L. siceraria can be equivalent to or greater than that of nongrafted watermelon (Bekhradi et al, 2011;Paroussi et al, 2007;Wimer et al, 2015a). Yetıs xır et al (2003) also reported that watermelon grafted onto L. siceraria produced higher yield than Cucurbita hybrids.…”

“…Recent studies in Washington have shown that grafting watermelon onto resistant rootstocks is a suitable management option for verticillium wilt (Buller et al, 2013;Dabirian et al, 2017;Wimer et al, 2015a). On a commercial scale, growers in Asia have been using grafting for several decades as a management strategy for biotic stresses such as fusarium wilt and verticillium wilt, and abiotic stresses such as soil salinity and cold stress in watermelon.…”

Fruit quality of seedless watermelon grafted onto squash rootstocks under different production systems

“…14 In a recent study in the northwestern USA, grafting did not affect SSC and lycopene content of fruit from seeded, diploid watermelon 'Sugar Baby' grafted onto Cucurbita maxima × C. moschata rootstocks. 18 However, in another study conducted in Greece, fruit from grafted seeded, diploid watermelon cultivars with C. maxima × C. moschata rootstocks demonstrated a potential delay in pre-harvest accumulation of sucrose while achieving higher lycopene content and improved flesh color during postharvest. 16 Grafting with the interspecific hybrid squash rootstock 'TZ148' also increased lycopene content of the seeded, diploid watermelon 'Pegasus', but the peak lycopene value of fruit from grafted plants was achieved later than in the non-grafted controls, 19 suggesting potential influence of rootstock on fruit development and ripening.…”

“…Studies of other quality factors such as soluble solids content (SSC), titratable acidity (TA) and lycopene content have produced conflicting results, most likely due to variations in environmental conditions and rootstock/scion combinations . In a recent study in the northwestern USA, grafting did not affect SSC and lycopene content of fruit from seeded, diploid watermelon ‘Sugar Baby’ grafted onto Cucurbita maxima × C. moschata rootstocks . However, in another study conducted in Greece, fruit from grafted seeded, diploid watermelon cultivars with C. maxima × C. moschata rootstocks demonstrated a potential delay in pre‐harvest accumulation of sucrose while achieving higher lycopene content and improved flesh color during postharvest .…”